The development of effective drug regimens against ovarian cancer and mesothelioma has proven extremely difficult. One promising approach that can improve patient outcome is the use of monoclonal antibodies (mAb). Mabuchi, et al., Curr Opin Obstet Gynecol (2010) 22(1):3-8.
Mesothelin (MSLN) was first identified in 1992 with the mAb K1 that was generated by the immunization of mice with human ovarian carcinoma (OVCAR-3) cells (Chang, et al., Int J. Cancer. (1992) 50:373-81). The mesothelin gene (MSLN) encodes a 71-kilodalton (kDa) precursor protein that is processed to a 40-kDa protein termed mesothelin, which is a glycosyl-phosphatidylinositol-anchored glycoprotein present on the cell surface (Chang, et al, Proc Natl Acad Sci USA (1996) 93:136-40). Mesothelin is a differentiation antigen whose expression is limited to mesothelial cells lining the body cavity. It is also overexpressed in a variety of cancers including mesothelioma, ovarian cancer, and pancreatic cancer (Hassan, et al., Eur J Cancer (2008) 44:46-53). In addition, mesothelin is expressed on the surface of many lung adenocarcinomas and other types of lung cancer (Ordonez, Am J Surg Pathol (2003) 27:1418-28; Ho, et al., Clin Cancer Res (2007) 13:1571-5). It has been shown that mesothelin is shed from tumor cells (Hellstrom, et al, Cancer Epidemiol Biomarkers Prev (2006) 15:1014-20.; Ho, et al., Cancer Epidemiol Biomarkers Prev (2006) 15:1751). Shed serum mesothelin has been approved by the US Food and Drug Administration as a diagnostic biomarker in malignant mesothelioma.
CA125 (also known as MUC16) was first identified in 1981 with OC125, a mAb that had been developed from mice immunized with human ovarian cancer cells (Bast, et al., J Clin Invest (1981) 68:1331-7). The first cDNA clones were reported in 2001 (Yin, et al., J Biol Chem (2001) 276:27371-5). CA125 is a very large membrane-bound cell surface mucin, with an average molecular weight between 2.5 and 5 million Daltons owing to its extensive glycosylation with both O-linked and N-linked oligosaccharides (O'Brien, et al., Tumour Biol (2002) 23:154-69). The peptide backbone of CA125 is composed of an N-terminal region with extensive Ser/Thr/Pro-rich tandem repeats (TR) of 156 amino acids each with both N- and O-glycosylation, a SEA (sea urchin, enterokinase, agrin) domain with high levels of O-glycosylation and a C-terminal region with a short cytoplasmic tail (O'Brien, et al., Tumour Biol (2001) 22:348-66). CA125 is used as a biomarker in ovarian cancer due to its high expression in ovarian carcinomas and release into the serum (Bast, et al., N Engl J Med (1983) 309:883-7).
A majority (88%) of mesotheliomas are also CA125 positive on the cell membrane (Bateman, et al., Histopathology (1997) 30:49-56; Attanoos, et al., Histopathology (2002) 40:237-44). The biological functions of CA125 are not fully understood. Recent studies on corneal epithelial cells have showed that CA125 is expressed by the ocular surface epithelia and localized on the tips of the surface microplicae (Blalock, et al., Invest Ophthalmol Vis Sci (2007) 48:4509-18). CA125 is a multifunctional molecule linked to the actin cytoskeleton. Rump and colleagues have shown that mesothelin binds to CA125 and that this interaction may mediate cell adhesion (Rump, et al., J Biol Chem (2004) 279:9190-8). Since mesothelin is present on peritoneal mesothelium, there may be an important role for the mesothelin-CA125 interaction in the metastic spread of ovarian cancer and mesothelioma in the peritoneal cavity. The mesothelin binding site on CA125 probably lies within the 156 amino acid TR units, indicating multimeric binding of mesothelin to CA125 (Scholler, et al., Cancer Lett. (2007) 247:130-6). It has been found that the very abundant N-glycans on CA125, presumably in the TR region, are required for binding to both glycosylated and non-glycosylated mesothelin (Gubbels, et al., Mol Cancer (2006) 5:50-65). We have recently identified a region (296-359) of 64 amino acids at the N-terminus of cell surface mesothelin as the minimum fragment for binding activity to CA125 (Kaneko, et al., J Biol Chem (2009) 284:3739-49).
Pastan and colleagues developed an immunotoxin (SS1P) that targets mesothelin expressing tumors. It contains a murine SS1 Fv fused to a 38-kDa fragment of Pseudomonas exotoxin A (PE38) (Pastan, et al., Nat Rev Cancer (2006) 6:559-65). Two Phase I clinical trials were completed at the National Cancer Institute (National Institutes of Health, Bethesda, Md.) and there was sufficient antitumor activity of SS1P to justify a Phase II trial. A chimeric antibody (MORAb-009) containing the same murine SS1 Fv for mesothelin was also developed and is currently being examined in a Phase II clinical trial for mesothelioma and pancreatic cancer (Hassan, et al., Cancer Immun (2007) 19:7:20).